The histamine-3 (H3) receptor is one of four histamine receptor subtypes (H1-H4), all of which are members of the larger G-protein-coupled receptor (GPCR) superfamily of receptors. The H3 receptor is predominantly expressed in the central nervous system. In the brain, it is located in regions associated with learning and memory such as the cerebral cortex, hippocampus and striatum. The H3 receptor acts as both auto- and hetero-receptor to regulate the release of histamine and other neurotransmitters. Within the cortex, the H3 receptor appears to directly modify GABA release from cortical interneurons. Antagonism of the H3 receptor produces a decrease in GABA release and disinhibition of the cortical cholinergic system, resulting in increased acetylcholine levels (Bacciottini, L. et al, Behavioral Brain Research, 124, 2001, 183-194). In addition to direct regulation of cholinergic neurotransmission, the H3 receptor has been shown to modulate the release of dopamine, serotonin and norepinephrine (Leurs, R., et al, Trends in Pharmacological Sciences, 19, 1998, 177-183). A postmortem study in humans suggests that a decrease in brain histamine levels may contribute to the cognitive decline which occurs in Alzheimer's disease, directly or through the cholinergic system (Panula, P., et al, Neuroscience, 82, 1998, 993-997). H3 agonists have been reported to impair memory in various tasks, such as object recognition, passive avoidance (Blandina, P., et al, British Journal of Pharmacology, 119(8), 1996, 1656-1664) and social olfactory memory (Prast, H., et al, 734, 1996, 316-318), whereas H3 antagonists have been reported to rescue impairments produced pharmacologically or genetically, i.e. Miyazaki, S., et al, Life Sciences, 61, 1997, 355-361; Meguro, K., et al, Pharmacology, Biochemistry and Behavior, 50, 1995, 321-325; Fox, G. B., et. al, Behavioral Brain Research, 131, 2002, 151-161; and Komater, V. A., et al, Psychopharmacology, 167, 2003, 363-372.
Accumulating neuroanatomical, neurochemical, pharmacological and behavioral data support the concept that H3 receptor antagonists may improve cognitive performance in disease states such as mild cognitive impairment and Alzheimer's disease and may have therapeutic value in the treatment of attention deficit hyperactivity disorder (ADHD), schizophrenia, obesity and sleep disorders.
Therefore, it is an object of this invention to provide compounds which are inhibitors of the H3 receptor and are useful as therapeutic agents in the treatment of a variety of central nervous system disorders related to or affected by the H3 receptor.
It is another object of this invention to provide therapeutic methods and pharmaceutical compositions useful for the treatment of central nervous system disorders related to or affected by the H3 receptor.
It is a feature of this invention that the compounds provided may also be useful to further study and elucidate the H3 receptor.